It is known that nuclear fuels irradiated in reactors are treated in plants especially constructed and equipped to recuperate the uranium not burned up and the plutonium, formed in the fuel. After the cutting or chemical decladding of the fuel rods, the core materials of the fuel rods are dissolved, generally in nitric acid, and uranium and plutonium are separated from the fission products by a series of extractions with selective organic agents as, in the case of the Purex process, tributyl phosphate (TBP), which is diluted in an organic solvent (aliphatic hydrocarbon of the kerosene type, or aromatic hydrocarbons)
This solvent is recovered at the end of each extraction cycle, decontaminated and used again many times. But finally, the mixture of TBP and kerosene is more and more degraded by the action of radiation and reagents and accumulates organic or organo-metallic degradation products, and its decontamination becomes after each cycle more and more incomplete. It is therefore necessary to renew the solvent and to reject the used solvent as waste.
It is the current practice to store the used solvent for a certain time in order to allow the fission products with a shorter half-life to decay before the used solvent is disposed of by incineration.
This way of working has several drawbacks:
1. The incineration leads to an atmospheric pollution as the phosphorus which is present in the waste is emitted into the atmosphere via a chimney in the form of phosphorus pentoxide which, in contact with the humidity of the air, is converted into phosphoric acid; thus, for instance, the incineration of a solvent of 30% TBP in kerosene at a rate of 20 l/h leads to the dispersion of 50 kg phosphoric acid per day. PA0 2. Even more severe is the fact that radionuclides, present in the solvent (plutonium and uranium included), are also released via the stack into the atmosphere; therefore, it is necessary to apply a pre-treatment to the solvent in order to decontaminate it. Such a decontamination can be effected by a process in which steam at 125.degree.-150.degree. C. is injected into the solvent. However, this leads to the undesired formation of a great volume of a low active aqueous waste and to an additional spreading of the activity. PA0 3. The temporary storage of the solvent waste requires a storage capacity which should be avoided for a safe and economic waste management. PA0 a. separation of the phosphoric acid ester from its organic diluent in such a way that the ester phase contains the degradation products and also the elements to be finally insolubilized; PA0 b. conversion of the phosphoric acid ester into phosphoric acids on one side and organic compounds mainly consisting of hydrocarbons on the other side; PA0 c. solidification of the liquid radioactive phosphoric acids in the form of inorganic phosphates, which can eventually be incorporated into a suitable matrix material. PA0 it requires no pretreatment resulting in a transfer or dilution of radioactivity; PA0 it can be realized with a reagent (phosphoric acid) produced by the process itself; PA0 it is non-polluting and no effluent is discharged, nor in the earth nor in the atmosphere, during the treatment of the solvent; PA0 it is an economic process in the sense that during the reactions involved in it enough fuel is produced for supplying the energy needed for the process; PA0 it allows a concentration of the radionuclides, present in the original solvent waste by a factor of 20 to 100; PA0 the inorganic concentrate obtained by this process can eventually be added to the aqueous medium-level or high-level liquid radioactive waste, in which it can play a favourable role for the insolubilization of the radionuclides during the solidification of the wastes, whatever be the selected solidification process.